Production declines in oil reservoirs take place as a result of depletion and/or damages by organic and inorganic species deposition (formation surface deposition, pore throat plugging, and re-entrainment into reservoir's fluids). The deposition of organic and inorganic species can be attributed to: (1) asphaltenes; (2) heavy paraffins; (3) carboxylate scale, carbonate scale, sulfate scale, or combinations of such scale in the forms of alkaline cations; and (4) finely dispersed clays particles (e.g., kaolinite, illite, smectite, or combinations) through perforation tunnels and gravel packs.
The pore throat diameter in oil reservoirs varies from 1 to 11 μm. Sandstone oil reservoirs, in particular, tend to be homogeneous with low permeability and porosity contrasts in any given single interval. Once precipitates or fine particles deposited within pores, they can not be extracted back into the reservoir by stimulation treatment. However, they can be pierced through into the other side of the plugged pores. The success of re-perforation in oil reservoirs is usually limited or short lived.
Depletion and asphaltenes precipitation in oil reservoirs can be averted by injecting, for instance, saline water to increase the reservoir's pressure above the saturation pressure and thus enhance oil recovery and prevent asphaltenes precipitation. Paraffins precipitation unlikely takes place if the reservoir's temperature is higher than the paraffins' cloud point temperatures.
Possible sources of saline water for injection operations in oil reservoirs include, for instance, aquifers saline water or oil-fields produced water. However, aquifers water could cause fine clays deposition depending on their formation rock while oil-fields produced water tend to cause at least calcium carboxylate and/or calcium carbonate scale deposition. Insufficient quantities and/or unacceptable qualities of each standalone source of saline water are always problematic in injection operations. Thus, such sources of saline water are most likely blended to sufficiently fill the reservoir voidage and fulfill the pressure support requirement.
The quality of saline water as strictly measured by its oil content and total suspended solids (TSS) is of significant importance since poor quality leads to injectivity reduction and wells plugging. The general parameters guidelines for the required quality of saline water to be injected in oil reservoirs are not to exceed: (1) 50 mg/L of oil content; and (2) 10 mg/L of TSS with 10 μm of particle size. Although providing meaningful parameters guidelines is important, however, such guidelines could obscure significant fundamental issues. Such issues could render saline water injection very expensive operations and cause irreversible reservoir damages.
True understanding of the nature and specific requirements for a given oil reservoir along with the chemistry of the designated saline water are a must for successful injection operations. Thus, this invention presents innovative and off the beaten path methods to provide an acceptable quality of saline water that is: (1) actually needed to steadily enhance oil production; and (2) technically and economically achievable.